1. Technical Field
The present invention relates to cognitive radio and, more particularly, to systems and methods for determining whether a given spectrum band is unoccupied.
2. Description of the Related Art
Cognitive radio (CR) that supports secondary (unlicensed) users to access licensed spectrum bands not being currently occupied can dramatically improve spectrum utilization. Since the licensed (primary) users are prior to the secondary users (SUs) in utilizing the spectrum, the secondary and opportunistic access to licensed spectrum bands is only allowed to have negligible probability of deteriorating the quality of service of the primary users (PUs). Spectrum sensing performed by the secondary users to detect the unoccupied spectrum bands, is an important step in meeting this requirement.
Several spectrum sensing schemes, such as matched-filter detection, energy detection, and cyclostationary detection, have been proposed and investigated. Among these sensing schemes, energy detection does not rely on any deterministic knowledge about the primary signals and has low complexity. However, energy detection entails considerable amount of sensing time at the low detection signal-to-noise ratio (SNR) level, e.g., the sensing time is inversely proportional to SNR2. To overcome this shortcoming, another sensing scheme, the sequential probability ratio test (SPRT), has been proposed for CR.
The SPRT has been widely used in many scientific and engineering fields since it was introduced in the 1940s. For given detection error probabilities, the SPRT requires a small average sample number for testing simple hypotheses. However, the SPRT-based sensing schemes proposed to date have several potential drawbacks: First, SPRT needs deterministic information or the statistical distribution of certain parameters of the primary signals. Acquiring such deterministic information or statistical distributions is practically difficult. Secondly, when the primary signals are taken from a finite alphabet, the test statistic of the SPRT based sensing scheme involves a special function, which incurs high implementation complexity. Thirdly, SPRT adopts the Wald's choice on the thresholds. However, the Wald's choice, which works well for the non-truncated SPRT, increases error probabilities when applied for the truncated SPRT.